Allow marking expressions to be defined as a column at first use
The implementation is straightforward: adding a forwarding node to identify (parts of) expressions that need to be defined, an reusing the existing code for the rest. The user-facing part is op.defineOnFirstUse
which returns the wrapped expression result.
I actually think this should be the default for get_scalefactor
(the definition will basically always end up in the right place for those: after the Filter
(if any) and before the weight definition)... any opinions?